1. Field of the Invention
The present invention relates to a rubber composition for a tire tread to be used for improving the gripping force of a studless tire, to a tire tread formed from such a rubber composition, and to a tire having such a tire tread.
2. Description of the Related Art
Predominant factors responsible for friction between a tread rubber and a road surface includes (1) hysteresis loss friction, (2) adhesion friction and (3) dig-out friction. When on a road surface having an extremely low friction coefficient such as a frozen road surface, the hysteresis loss friction (1) is extremely low, and friction due to items (2), (3) are predominant.
Although a spike tire can provide an excellent gripping force on a frozen road surface since it produces a large dig-out friction force, there has recently been a tendency to inhibit the use of spike tires so as to prevent pollution caused by powdery dust. Thus, studless tires with no spikes have rapidly prevailed.
Since the dig-out frictional force depends on the tread pattern or surface form of the tread, the studless tire has also been devised to improve the dig-out frictional force thereof, for example, by incorporating therein foamed rubber or organic fiber, thereby increasing unevenness on the tread surface. However, the dig-out frictional force of even a studless tire thus devised is low as compared with the spike tire and, hence, the gripping force thereof is inferior to that of the spike tire. Particularly when a frozen road surface is ground smooth by idle rotation of the tire upon the starting of a vehicle, the gripping force of the studless tire is degraded.
When on a road surface having an extremely low friction coefficient, such as a frozen road surface, the adhesion friction as well as the dig-out friction contributes to an improvement in the gripping force. It is important in this instance to increase the area of contact between the road surface and the tire by eliminating hydroplaning formed due to friction by thawing of the frozen road surface, in order to improve the adhesion friction. The unevenness on the tread surface not only improves the dig-out frictional force but also has the effect of eliminating hydroplaning to increase the adhesion friction. Further, the characteristics of the rubber itself give an effect on the adhesion frictional force. For instance, the adhesion frictional force can be increased by making the rubber material soft thereby increasing the area of contact with a road surface.
Although various proposals have been made to improve the gripping force of a studless tire by increasing the adhesional frictional force, none of them has been satisfactory and further improvements have been demanded.
The present invention has been achieved in view of the foregoing technical background and an object thereof is to provide a rubber composition for tire tread which provides for a studless tire with an improved gripping force.
The present inventors have made various studies for increasing the adhesion frictional force of a tire on a frozen road surface and, as a result, have developed the present invention by determining that the frictional force can be increased by adding a calcined clay, e.g. amorphous metakaolin resulting from calcination at a high temperature of about 600.degree. C.